Planar solar concentrator power module

ABSTRACT

A planar concentrator solar power module has a planar base, an aligned array of linear photovoltaic cell circuits on the base and an array of linear Fresnel lenses or linear mirrors for directing focused solar radiation on the aligned array of linear photovoltaic cell circuits. The cell circuits are mounted on a back panel which may be a metal back plate. The cell circuit area is less than a total area of the module. Each linear lens or linear mirror has a length greater than a length of the adjacent cell circuit. The cell circuit may have cells mounted in shingle fashion to form a shingled-cell circuit. In an alternative module, linear extrusions on the circuit element have faces for mounting the linear mirrors for deflecting sun rays impinging on each mirror onto the shingled-cells. The linear extrusions are side-wall and inner extrusions with triangular cross-sections. The circuit backplate is encapsulated by lamination for weather protection. The planar module is generally rectangular with alternating rows of linear cell circuits and linear lenses or linear mirrors.

This application claims the benefit of U.S. Provisional Application No.60/374,808 filed Apr. 24, 2002 and U.S. Provisional Application No.60/391,122 filed Jun. 24, 2002.

BACKGROUND OF THE INVENTION

Solar cells generate electricity but at a cost which is too high tocompete with electricity from the electric power company. It isgenerally acknowledged that the solar panel cost will have to drop toapproximately $1 to $2 per installed Watt before solar cells can competein this large potential market. Today's cost for solar panels is in the$6 to $7 per Watt range. Three different approaches have been pursued inattempts to resolve this cost problem.

The conventional approach is to use large silicon solar cells tiled inplanar modules where the cell area represents over 80% of the totalpanel area. The cells in this approach can be single crystal or largegrain polycrystalline cells. This approach represents over 90% of themarket but the cost of this approach has bottomed out and no furthercost reductions are expected.

The second approach is based on the assumption that the cost of siliconwafers is too high and one needs to make low cost thin film cells. Theargument is that paint is cheap and that maybe a way can be found tomake paints generate electricity. This thin film approach includesamorphous silicon and small grain-size polycrystalline materials likeCuInSe2 and CdTe. The problem with this approach has been thatdestroying the crystal material degrades solar cell performance. Todate, this approach has not yielded modules costing less than $8 perWatt.

The third approach is based on concentrating the sunlight onto smallsingle crystal cells using larger inexpensive plastic lenses or metalmirrors. This approach allows more efficient cells to be used and makesgood technical sense. However, the problems with this approach are nottechnical but instead relate to business and politics. Solving thebusiness problems inherent in this approach is the focus of thisinvention.

Serious attempts to develop solar concentrator photovoltaic systems canagain be divided into three parts. First, attempts have been made to usepoint focus lenses and 30% efficient cells where the systems operate athigh concentration ratios, e.g. approximately 500 suns. The problem hereis not with the technology. The various components work, and systemshave been demonstrated. The problem here is that the investment requiredto create positive cash flow is too large. Large companies will not takethe risk and small companies do not have the resources and thegovernment is not helping. The 30% cells are not being manufactured andinvestment is required here. Furthermore, trackers with the requiredaccuracy are not being manufactured. Again investment is required.Investment is also required for the thermal management and lenselements. Finally, these systems are not cost effective unless made inlarge sizes and in large volumes and there are no intermediate marketsother than the utility scale market.

The second approach to solar concentrators involves the use of archedlinear Fresnel lenses and linear silicon solar cell circuits. Thesesystems are designed to operate at approximately 20 suns. This is also atechnically proven approach but this approach also suffers from theinvestment problem. Here, investment is again required for speciallenses, trackers, and thermal management systems. Here, the plan is thatthe cells will be available from the cell suppliers who make planararrays. However, this presents two problems. The first problem is thatthe planar cells have to be significantly modified to operate at 20suns. The second problem is that the planar cell suppliers are notmotivated to cooperate. For example, suppose that the concentratorapproach proves to be cheaper and the market expands by three times. Theproblem for the planar cell suppliers is that their part will actuallyshrink by 3/20 times. Again, these systems are not cost effective unlessmade in large sizes and in large volumes and there are no intermediatemarkets other than the utility scale market.

The third approach to solar concentrator systems was initiated by theplanar module manufactures. Realizing that if their one sun planarmodule were operated at 1.5 suns, they could produce 1.5 times morepower and consequently reduce the cost of solar electricity by 1.5times, they built a system using edge mirrors to deflect sunlight fromthe edge areas onto their panels. Unfortunately, this approach wastechnically naive. The problem encountered was that the modules thenabsorbed 1.5 times more energy and there was no provision to remove theadditional heat. This then affected the module lifetime.

Solar concentrators require very high investments to scale up productionof a new concentrator cell. The investment required for manufacturingscale-up versions of a new cell is prohibitive. Another problem thatneeds to be solved is the cell-interconnect problem.

There is a need for a solar concentrator module that is a retrofit for aplanar module and that is easier and cheaper to make. The businessinfrastructure for trackers and lenses should already be in-place. Theheat load should be easily manageable. Investment requirements should bemanageable and it should not threaten existing cell suppliers. Cells tobe used should be available with very minor changes relative to planarcells. Therefore, low cost cells should be available from today's cellsuppliers. Finally, it should be usable in early existing markets inorder to allow early positive cash flow.

SUMMARY OF THE INVENTION

The present invention addresses and resolves the above needs. FIGS. 1,2, and 3 show a preferred planar concentrator solar power module. Theunit depicted measures, but is not limited to, 25″ by 40″ by 3.25″ deep.The size depicted is exemplary and is similar to 75 W planar modulesmanufactured by Siemens, Kyocera, and Solarex. All of these modulesmeasure approximately 25″ by 40″ and produce about the same power. Othersizes are also within the scope of this invention.

The conventional planar modules consist of large silicon cellssandwiched between plastic sheets with a glass front plate surrounded bya 2″ thick metal frame, for example aluminum frame, for rigidity. Ourpreferred planar concentrator solar module consists of a back metalsheet upon which linear silicon cell circuits are mounted. In theembodiment depicted, there are plural circuits, as for example but notlimited to, 6 circuits containing cells approximately 1.3″ or 1.2″ wide.The circuit separation is, for example, 4″ or 3.6″. Therefore, the cellarea represents one third of the total module area resulting in a majorcost reduction for cells.

In the preferred module, as an example, a 3.25″ thick metal frame, forexample, aluminum frame, surrounds the module with the cells mounted onthe back panel. A lens array is mounted on a glass sheet forming thefront side of the planar concentrator solar module. There are, forexample, 6 linear Fresnel lenses on this front sheet with each lensbeing, for example, about 3.6″ or about 4″ wide and aligned such thatthe solar rays from each lens impinge on a linear power circuit. In theexemplary case, there are 6 lenses and 6 aligned circuits. Otherconfigurations with more or less lenses and more or less circuits arewithin the scope of this invention. Alternatively, several of thesegoals may be accomplished using linear extruded elements with mirroredfaces.

The modules are manufactured at a cost below today's module cost. Aprice in the $3 to $4 per Watt range, below the present $6 to $7 perWatt range, and subsequently, at the $1 to $2 per Watt range, istargeted. A successful 3-sun module results in a larger investmentenabling the $1 to $2 per Watt target to be eventually achieved with 30%efficient concentrator cells operating at higher concentration ratios.

Applicants have previously described a similar linear lens and linearcircuit configuration for a different set of applications. In thatdisclosure, we noted that the preferred pointing requirement for thisdesign is very broad, being greater than +/−5 degrees in bothdirections. One preferred embodiment of that device incorporates Fresnellenses and silicon solar cells. For example, a 6″×8″ solar batterycharger, about 2.5″ thick may yield about 4 W. It can collapse to about½″ thickness for easy transportation. Angle tolerance along the circuitlength/direction is about +/−20 degrees which corresponds to over twohours between alignments. For example, if one sets the circuits verticalin the early morning and late afternoon, and horizontal in the midday,the device will require 3 alignments in an eight-hour period. 30%efficient cells of about 8″×12″ yield 16 W.

Advantages of the present planar solar concentrator power moduleinclude, but are not limited to, the following:

1. The cells are mounted in rows on a metal back plate with the rowsclose enough to each other that the heat can spread in the back plate sothat the air cooling area is similar to that of the standard planarmodule.

2. The cells used may be obtained from several different planar cellsuppliers requiring only a minor change in grid design, for examplechanges in size and front metal pattern, to operate at 2-3 suns.

3. The linear circuit assembly may be automated leading to a furthercost savings relative to traditional planar module assembly, which ispresently, labor intensive.

4. The thermal management is easily handled. The heat spreads in theback metal plate such that the air contact area for heat removal is thesame as the lens area. This means that the heat removal is equivalent tothe planar module case.

5. There are already several Fresnel lens suppliers. The concentrationratio is low and not technically challenging. The lenses should be lowcost when made in high volume.

6. The modules are designed such that the elongated dimension, the cellrows, and the linear concentrator elements are oriented along theNorth-South direction with the linear concentrator elements being longerthan the cell rows so that the modules can be mounted on single axistrackers without requiring seasonal adjustments.

7. The modules operate at low concentration on simple single-axistrackers requiring pointing tolerance of no less than +/−2 degrees.

8. This unit resembles smaller units in design and assembly and can beused in different devices such as, but not limited to, solar batterychargers for cell phones, digital cameras, PDAs such as, but not limitedto, the Palm Pilot, laptop computers, etc.

9. The planar modules and the concentrator modules are similar in formand function.

10. The concentrator module unit operates at two-suns or three-sunsusing one-half or one-third the silicon solar cell area relative totraditional planar panels thus leading to a major cost savings.

11. The cells used can be obtained from several different planar cellsuppliers requiring only a minor change to operate at two-suns orthree-suns.

12. The concentration ratio is low and not technically challenging.Extruded linear elements with mirrored faces are low cost.

13. The tracking accuracy requirement is minimal. This means thatcommercially available liquid refrigerant trackers from Zomeworks can beused.

14. The Zomeworks trackers are presently used with planar solar modulesfor farm irrigation systems. This means that there is an immediateintermediate market with marketing channels already established.

15. The fact that this unit resembles a traditional planar solar modulewill lead to easy customer acceptance. Thereafter, it would lead to aneven more lower cost, higher concentration ratio systems.

These and further and other objects and features of the invention areapparent in the disclosure, which includes the above and ongoing writtenspecification, with the claims and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of the planar solar concentrator power module ofthe present invention.

FIG. 2 shows a cross section through the planar solar concentrator powermodule of FIG. 1.

FIG. 3 shows a blow up section from FIG. 2 with a single lens andcircuit element in more detail.

FIG. 4 is a side view of a smaller planar solar concentrator powermodule with two lens and circuit elements.

FIG. 5 shows a top view in which the lenses magnify the circuits of thepower module of FIG. 4.

FIG. 6 shows a section of the back metal sheet with embossed terraces infabricated linear circuits.

FIG. 7A and 7B shows a shingle circuit in which the cells are mounted inthe terraces of FIG. 6 in a shingle fashion.

FIG. 8 shows a perspective view of a planar solar concentrator powermodule. Note however, as shown in FIG. 5, the lens effect is merelyexemplary.

FIG. 9 shows several planar solar concentrator power-modules mounted ona solar tracker.

FIG. 10 shows a 3D view of the planar solar concentrator power module ofthe present invention with mirror elements.

FIG. 11 shows a cross-section through the planar solar concentratorpower module of FIG. 10 at A-A.

FIG. 12 shows a blow up section from FIG. 11 showing a circuit elementand two types of linear extrusions with mirrored faces.

FIG. 13A shows a commercial planar cell.

FIG. 13B shows the cell of FIG. 13A cut up into four 2×concentrator-cells.

FIG. 13C shows the cells reassembled into a shingled-cell circuitelement.

FIG. 14 shows two shingled-cell circuit elements mounted on the heatspreader metal back-plate with a stress relief ribbon bond between thecircuit elements.

FIG. 15 is a top view of two series connected standard planar cells.

FIG. 16 is a top view of two series connected standard planar cells cutin half and mounted in a 2-sun concentrator mirror module.

FIG. 17 is an end view of the cell and module of FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1, 2, 3, and 8 show a preferred planar concentrator solar powermodule 1. FIG. 1 shows a top view of the planar solar concentrator powermodule 1 of the present invention. An array 3 of linear Fresnel lenses 5produces lines of focused solar radiation that fall on an aligned arrayof linear photovoltaic power circuits. FIG. 2 shows a cross sectionthrough the planar solar concentrator power module 1 of FIG. 1. Thecross section is perpendicular to the focal lines produced by the lensesand perpendicular to the circuit length dimension. FIG. 3 shows a blowup section from FIG. 2 showing a single lens 21 and circuit element 23in more detail.

The preferred unit depicted is exemplary with dimensions not limited tothe following. For example, the preferred unit may measure 25″ by 40″ by3.25″ deep. The size depicted is exemplary and is similar to 75 W planarmodules manufactured by Siemens, Kyocera, and Solarex. All of thesemodules measure approximately 25″ by 40″ and produce about the samepower. The planar modules consist of large silicon cells sandwichedbetween plastic sheets with a glass front plate surrounded by a 2″ thickmetal frame, for example aluminum frame for rigidity. Other dimensionsare well within the scope of this invention.

The preferred planar concentrator solar module consists of a back panelof metal sheet 25 upon which linear silicon cell circuits 7 are mounted.In the exemplary embodiment depicted, there are, for example but notlimited to, 6 lenses and 6 aligned circuits containing, for example,cells 23 about 1.3″ or about 1.2″ wide. The circuit separation is, forexample, about 4″ or 3.6″. Therefore, the cell area represents one thirdof the total module area resulting in a major cost reduction for cells.

In the preferred module, for example, about a 3.25″ thick metal frame 9,for example aluminum frame, surrounds the module 1 with the cells 23 ofthe cell circuits 7 mounted on the back panel 25. A lens array 3 ismounted on a glass front sheet 27 forming the front side of the planarconcentrator solar module 1. There are, for example, 6 linear Fresnellenses 5 on this front sheet 27 with each lens 21 being, for example, 4″wide and aligned such that the solar rays from each lens 21 impinge on alinear cell circuit 7 connected to a power circuit assembly 11 with+/−terminals 13. In the FIG. 1 example, there are 6 lenses and 6 alignedcircuits. Other lens and circuit combinations are within the scope ofthis invention.

A preferred lens 5 has, for example, a lens width and circuit spacing ofapproximately 4″ or 3.6″. Given a lens width of 4″ or 3.6″, the shortestreasonably achievable focal length will be approximately 4″ or 3.6″respectively. If, then, the lens is set to a cell spacing, for example,at 3.25″ or 3″, the focal line on the circuit will be approximately0.75″ or 0.7″ wide. For a cell width of 1.3″ or 1.2″, there will be anilluminated region of 0.75″ or 0.7″ wide and dark bands on either sideof (1.3−0.75)/2=0.28″ or (1.2−0.7)/2=0.25″ width. These dark regionswill allow a tracking band of tan⁻¹(0.28/3.25)=+/−5 ortan⁻¹(0.25/3)=+/−5 degrees wide. We choose this tracking band toleranceprecisely because this is the tracking tolerance of the commerciallyavailable Zomeworks tracker. The same applies for the 2× mirror module.

A lens width of 4″ provides several advantages, Firstly, the 4″ widthallows for a panel thickness of 3.25″, not too much thicker than astandard planar solar panel (2″). Secondly, the 4″ width is advantageousfor thermal management. The lenses 5 concentrate the solar energy intothe circuits 7. The waste heat then is transferred to the metal backplate 25. It then spreads laterally through the metal such that themetal plate temperature is nearly uniform. If the spacing betweencircuits is too large, the heat will not spread uniformly and thecircuits will run too hot. A 4″ or 3.6″ spacing does allow for a uniformplate temperature with a reasonably thin and light back metal plate. Thesame also applies for the 2× mirror module.

The linear Fresnel lenses 5 provide several advantages. The firstrelates to seasonal alignment. The sun's midday position moves north andsouth+/−23 degrees from summer to winter. This movement is accommodatedusing a linear lens 21 by making the lens length longer than the circuit23 length and aligning the lens focal line in the north/south direction(see, for example, FIG. 1). In the present module 1, the circuits 7 are,for example but not limited to, about 1.4″ or 1.3″ shorter than the lens5 at both ends 15, 17. This gives a tracking tolerance in thenorth/south direction of tan⁻¹(1.4/3.25)=+/−23 degrees or tan⁻¹(1.3/3)=+/−23 degrees, respectively.

The second reason for the having the linear Fresnel lenses 5 is thatthey lead to linear power circuits 7 and a linear power circuit assembly37 (FIG. 7) can be automated.

FIGS. 4 and 5 are examples of a smaller planar solar concentrator powermodule 31 with two lens and circuit elements 33. The FIG. 4 power module31 shows the unit from a side view whereas FIG. 5 shows the lensesmagnifying the circuits from the top view. The FIG. 5 power module 31 ispresented to show the lens magnification effect.

FIGS. 6 and 7 show one example of fabricating the linear circuits. FIG.6 shows a section of the metal back sheet 25 with embossed terraces 35.FIG. 7 shows the cells mounted in these terraces in a shingle fashion 37with the back edge 39 of one cell 41 overlapping the front edge 43 ofthe previously placed cell 45.

The cell design is typical for solar cells having complete metaldeposited on the backside with a grid pattern on the front. In thiscase, the grid lines run in the circuit direction and connect to a busbar on one edge of the top of the cell. The bus electrically connects tothe back metal of the next cell. In this way, a front to back seriesconnection is made between cells to make shingle circuits.

As shown in FIG. 1, there are circuit terminal pads 49 at the end ofeach circuit. Insulated metal elements, such as but not limited to,ribbons 47 and 51 connect these circuits to plus and minus panelterminals 13.

As shown in FIG. 3, an insulating film 53 (as for example paint) may beplaced over the metal back plate 25 so that the circuit 7 is in goodthermal contact with the metal back plate but not in electrical contactwith the back plate.

We have previously described shingle circuits in the context ofthermophotovoltaics. An important consideration for shingle circuits isthat the coefficient of thermal expansion (CTE) of the metal back plateshould be matched with the coefficient of thermal expansion of the cellsso that the electrical connecting bonds between cells are not pulledapart as the module heats or cools. With matched CTES, the cells andmetal back plate expand and contract together with changes intemperature. In the case of silicon solar cells, an appropriate metalmay be, for example but not limited to, alloy 42 (Fe 58%, Ni 42%).

The shingle circuit assembly 37 depicted here is not the only way inwhich linear circuits 7 can be fabricated. Other fabrication methods arewithin the scope of this invention. For example, ribbon leads can be runfrom the back of one cell to the front of the next cell. Loops in theribbon connections will allow for differences in CTEs so that analuminum back plate may then be used. This assembly procedure can beautomated although with more steps than for the shingle circuitfabrication sequence. Either circuit type, and assembly process, fallswithin the scope of the present invention.

FIG. 8 shows a perspective view of a planar solar concentrator powermodule 1. Note however, as shown in FIG. 5, the lens effect is notrealistically depicted.

FIG. 9 shows several planar solar concentrator power-modules 1 mountedon a solar tracker 55. The present planar solar concentrator powermodule may be provided in solar powered water pumping devices which isused for irrigation worldwide. The advantages of the present moduledesign include, but are not limited to, cheaper costs of productionbecause it uses less expensive single crystal material, and easyavailability and adaptability for immediate replacements in severaldifferent applications.

A preferred planar concentrator photovoltaic module comprises a planararray of linear Fresnel lenses in front of and aligned with a planararray of linear circuits. The circuits are mounted on a metal sheetallowing for heat spreading and heat removal.

The planar concentrator photovoltaic module further comprises lenseswhich are longer than the circuits such that the acceptance angle forsunlight in the length dimension is greater than or equal to 20 degreesallowing for polar axis tracking without seasonal adjustments.

The planar concentrator photovoltaic module comprises lens having widthsabout three times larger than the width of the cell and circuit. Theimage from the lens underfills the cells with dark bands on the cells oneither side of the image allowing for the acceptance angle for sunlightin the width dimension to be greater than or equal to about 5 degrees.

The planar concentrator photovoltaic module further comprises a metalsheet upon which the circuits are mounted. The metal sheet has acoefficient of thermal expansion compatible with the coefficient ofthermal expansion of the cells. The metal sheet is embossed withterraces upon which the cells are mounted in a linear shingle circuit.

FIGS. 10, 11, and 12 show the planar concentrator solar power module ofthe present invention. FIG. 10 shows alternating rows of linearshingled-cells and linear mirror elements where the mirrors deflect sunrays down to the cells. FIG. 11 shows a cross-section through the planarsolar concentrator power module of FIG. 10 at A-A. The cross-section isperpendicular to the rows of cells and mirrors. FIG. 12 shows a blow-upsection from FIG. 11 showing a circuit element and, as examples, twotypes of linear extrusions: the module side wall extrusion and the innerextrusions with triangular cross-sections.

A preferred unit measures, for example, about 20″ by 44″ by 3.25″ deep,preferably 21″ by 47″ by 3.25″ deep. The size depicted is exemplary andis similar to 75 W one-sun planar modules manufactured by BP Solar,Siemens, Kyocera, and Solarex. All of these one-sun modules measureapproximately 20″ by 44″ or about 21″ by 47″ and produce about the samepower. The one-sun planar modules consist of large silicon cellssandwiched between plastic sheets with a glass front plate surrounded bya 2″ thick aluminum frame for rigidity.

The present planar concentrator solar module 100 consists of a backpanel 103, preferably of metal sheet, upon which linear silicon-cellcircuits 105 are mounted. In the example shown, there are four circuitscontaining cells 107 approximately 2.5″ wide. The preferred circuitseparation is approximately 5″. Therefore, the cell area represents onehalf of the total module area resulting in a major cost reduction forcells. In the exemplary module, an approximately 3.25″ thick aluminumframe 122 with end plates 123 surrounds the module with the cellsmounted on the back panel 103.

Mirrors 109 are located between the rows of linear silicon-cell circuits105. Referring to FIGS. 11 and 12, the mirrors 109 are mounted on thefaces 111, 113 of the exemplary two types of linear extrusions 115, 117.Sun rays hitting each mirror 109 are deflected down onto the cells 107.The two types of linear extrusions are the module side-wall extrusions115, and the inner extrusions with triangular cross-section 117.

The present module shown is mechanically assembled as follows. Cells 107are first mounted on the metal heat spreader plate 103. For low cost,the cells 107 are mounted in shingled-cell circuits 105. The heatspreader plate 103 is captured in slots 119 in the side wall extrusions115. There are fastener holes 121 running from end to end in both typesof extrusions 115, 17. Fasteners such as, but not limited to, sheetmetal screws 120 (FIG. 10) running through holes 121 in end plates 123(FIG. 10) connect all of the elements together.

As noted above a problem for solar concentrators has been the highinvestment required to scale up production of a new concentrator cell.Therefore, it is desirable to use planar solar cells that are already inhigh volume production at low cost. FIGS. 13A-C show how to produce thesolar concentrator cells 130 at low costs by simply cutting a commercialplanar cell 132 into four parts. FIG. 13A shows a commercial planar cell132. FIG. 13B shows the cell of FIG. 13A cut up into four 2×concentrator-cells 130. FIG. 13C shows the cells reassembled into ashingled-cell circuit element 134.

Another problem solved by the invention is the cell-interconnects forconcentrator modules. A standard one-sun planar module typically hasthirty-six 5″ square cells. While the present concentrator module hashalf the cell area, each cell is one quarter the area of a planarone-sun cell. Therefore, there are twice the number of cells or 72cells. If stitch ribbon bonds from cell to cell are used, as is typicalfor planar modules, twice the number of bonds will be required, whichwill increase assembly cost. This problem is resolved by makingshingled-cell circuit elements 134 as shown in FIGS. 13B, 13C, and 14.

FIG. 13C shows a top view of an exemplary shingled-cell element 134containing four cells. FIG. 14 shows the side view of a circuit row 136containing two shingled-cell circuit elements 134 mounted on the heatspreader metal back-plate 138 with stress relief ribbon bonds 40 betweencircuit elements 134. In a shingled-cell element 134, cells 142 aremounted in a shingle fashion with the back edge 144 of one celloverlapping the front edge 146 of the previously placed cell. Theshingled-cell design is used for solar cells having a complete metaldeposited on the backside with a grid and bus pattern on the front. Inthis case, the bus runs in the circuit direction. The bus electricallyconnects to the back metal of the next cell. In this way, a front toback series connection is made between cells to make shingle circuitelement. There are half as many bonds used for shingled-cell circuits aswhen stitch ribbons are used from cell to cell.

As pointed out above, an important consideration for shingle circuits isthat the coefficient of thermal expansion (CTE) of the metal back plateshould be matched with the coefficient of thermal expansion of the cellsso that the electrical connecting bonds between cells are not pulledapart as the module heats or cools. With matched CTEs, the cells andmetal back plate expand and contract together with changes intemperature. In the case of silicon solar cells, an appropriate metalmay be alloy-42 (Fe 58%, Ni 42%).

It is also possible to use shingled-cell elements even if there is not aperfect CTE match between the cells and the metal backing plate. This isdone by providing flexibility in the adhesive 148 used to bond the cells142 to the metal back plate and by providing periodic stress reliefribbon bonds 140. FIG. 14 shows a stress relief ribbon bond 140 betweentwo shingled-cell circuit elements 134. Cells in the back panel may beprotected by lamination with a transparent encapsulant 149. Laminationmay be, but is not limited to, a transparent plastic sheet covered by atransparent Teflon sheet or a glass plate as used in the planar modules.

Another problem is to determine how often stress relief ribbon bonds 140are required. The present invention provides a solution to this problem.For example, 4″ long Si cells have been mounted with flexible thermallyconductive adhesive to aluminum plates and operated for ten years in asolar concentrator prototype without failure. The CTE difference betweenaluminum and silicon is (22−4)×10⁻⁶ per C. In the present inventionusing a carbon steel back plate with a CTE difference of (11−4)×10⁻⁶ perC, or 2.5 times less, uniquely allows for the use of a shingled-cellelement 2.5 times longer than 4″ or 10″ long, as shown in FIG. 14.

The invention addresses the high-cost problems for solar concentratorsrequiring scale up production of new concentrator cells. FIGS. 15, 16,and 17 show how to use planar solar cells already in high volumeproduction in the present modules. The present solar concentrator cellsare formed by cutting up commercial planar cells into halves.

In the 2× mirror module, as shown in FIGS. 15, 16, and 17, planar cells150 are cut in half 152, separated by mirror elements 166 and operatedat 2× concentration with the mirrors. Thus half the number of planarcells 154, 156 are used to form a module 160 with the same power. Also,a further advantage is that half the number of ribbon bonds 158 arerequired. Ribbon bonding techniques are similar to the planar modulerequiring no additional equipment nor manpower for mirror modulefabrication.

The metal back plate 162 with linear circuits 164 are formed as shown inFIGS. 16 and 17. After the cells 150 are cut and ribbon bonded 158 frontto back, the circuit strings 168 are bonded to the metal back plate 162with thermally conductive epoxy 170. The circuits 164 are electricallyisolated from the metal back plate. After the circuits 164 are bonded tothe metal back plate 162 and wired together, the cells in the back panelare then protected by lamination with a transparent encapsulant.Lamination may be, but is not limited to, a transparent plastic sheetcovered by a transparent Teflon sheet or a glass plate as used in theplanar modules, as shown at 149 in FIG. 14.

The interconnect procedures for connecting ribbons to cells front toback is similar to that used in the planar modules. The ribbonconnections are labor intensive and therefore alternative ways may beused for the interconnections. For example, shingled cell circuits aremade as shown in FIGS. 13A-C and 14, which enables a faster cell-circuitassembly using automated equipment.

The present cell and mirror height dimensions are preferably between 2″and 3″, more preferably 2.5″. That allows for a panel thickness of about3.25″, not too much thicker than a standard planar solar panel withthickness of about 2″. Also, it provides optimal thermal management. Themirrors concentrate the solar energy into the circuits. The waste heatthen is transferred to the metal back plate. It then spreads laterallythrough the metal such that the metal plate temperature is nearlyuniform. If the spacing between circuits is too large, the heat will notspread uniformly and the circuits will run too hot. A preferred spacingof about 5″, or about 2.5″ from cell edge to the next cell edge, allowsfor a uniform plate temperature with a reasonably thin and light backmetal plate.

Linear circuits and mirrors provide several advantages. Firstly,aluminum extrusions and linear shingled circuits are easy to make.Secondly, it provides for optimal seasonal alignment. The sun middayposition moves north and south+/−23° from summer to winter. Thismovement is accommodated using a linear mirror by making the mirrorlength longer than the circuit length and aligning the mirror focal linein the north/south direction. The present module circuits are about 1.4″shorter than the mirrors at both ends which uniquely gives a trackingtolerance in north/south direction of tan⁻¹(1.4/3.25)=+/−23°.Preferably, providing circuits shorter by about 1.2″ than the mirrors atboth ends gives tracking tolerance in the north/south direction oftan⁻¹(1.2/2.5)=+/−25°.

The invention also provides pointing tolerance in the east/westdirection. If the mirror tilt angle referenced to the normal from thecell plane is 30°, then all of the rays reflected by the mirrors willfall on the cells as long as the module is precisely pointed at the sun.This invention provides a pointing tolerance of approximately +/−2°translating to a mirror tilt off-normal angle of approximately 26°.

The present invention allows for the manufacture of modules at a costbelow today's module cost. Target prices are in about the $3 to $4 perWatt range, preferably at about the $1 to $2 per Watt cost range, wellbelow the $6 to $7 per Watt price range of present modules.

While the invention has been described with reference to specificembodiments, modifications and variations of the invention may beconstructed without departing from the scope of the invention, which isdefined in the following claims.

1. A method of assembling a planar concentrator solar power modulecomprising obtaining existing, readily available commercial planar solarcells, each solar cell having a continuous grid electrode, cutting thesolar cells in segments, each resulting segment comprising a portion ofthe grid electrode, mounting the divided cells in precisely spaced rowson a metal beat spreader back plate and forming a circuit element,connecting the cells in series to form linear circuit rows, mountingflat, linear mirrors on the plate, alternating the linear circuit rowsand the flat, linear mirrors in the circuit element, deflecting sun rayswith the linear mirrors on to the linear circuit rows, concentratingsolar energy into the linear circuit rows, transferring waste heat tothe metal heat spreader back plate spreading the waste heat laterallythrough the metal plate so that a temperature of the metal plate isnearly uniform, and providing optimal thermal energy management.
 2. Themethod of claim 1, further comprising transferring waste heat generatedfrom the concentrating solar energy to the metal back plate, spreadingthe waste heat laterally through the metal plate and causing atemperature of the metal plate to be uniform.
 3. The method of claim 1,wherein the mounting the cells on the metal plate comprises providingadequate spacing between alternating circuits and allowing a temperatureof the metal back plate to be uniform.
 4. The method of claim 1, furthercomprising mounting linear extrusions on the metal back plate andmounting the flat, linear mirrors on faces of the linear extrusions andmounting the linear circuit rows between the mirrors.
 5. The method ofclaim 1, further comprising allowing for optimal seasonal alignment byproviding flat, linear mirrors longer than the linear circuit rows,aligning the mirror focal line in a north/south direction and giving atracking tolerance in the north/south direction greater than or equal to20 degrees.
 6. The method of claim 5, further comprising allowing for apointing tolerance in a east/west direction by providing a mirror tiltoff-normal angle corresponding to a tracking movement of the sun.
 7. Themethod of claim 6, wherein the off-normal angle is about 26°.
 8. Themethod of claim 5, further comprising mounting the module on asingle-axis tracker, wherein seasonal adjustment is not required.
 9. Themethod of claim 1, wherein the assembly steps are automated.